The present invention relates to a photosensitive composition used in insulating protective coating films for printed circuit boards and the like, and cured articles thereof.
A protective film made of polyimide or polyester has widely been used as insulating protective coating films for circuit boards. When this protective film is formed on a circuit board, an adhesive is applied on one surface of the protective film. After perforating the portion which corresponds to a terminal connection when the protective film is formed on the circuit board by means of punching, the protective film and the circuit board are laid one upon the other by hand while positioning them and then are bonded to each other at high temperature under high pressure using a heat plate.
Polyimide and polyester are suited for protection of wiring because they are pliable to some extent. However, a film must be perforated in advance by means of punching to lead a wiring terminal on the circuit board described above. Therefore, complicated wiring makes it difficult to execute positioning of the film with wiring on the board. Since the cost of the equipment such as a heat plate is high and an adhesive must be used, the cost becomes higher. Also, there was such a problem in that smearing occurs during the punching and oozing of the adhesive occurs during the pressing.
Even if a protective film having excellent heat resistance and electrical insulating properties is used as the insulating protective coating film in such a manner, the performance of the adhesive sometimes exerts an adverse effect on the insulating protective coating film, thus lowering the heat resistance and electrical insulating properties of the insulating protective coating film when the circuit board and the protective film are bonded via the adhesive.
Therefore, a method of forming the insulating protective coating film on the circuit board by means of a printing method using cover lay ink (hereinafter referred to as a xe2x80x9ccover lay ink methodxe2x80x9d) has recently been studied.
As an example of cover lay ink, for example, Japanese Patent Application, First Publication No. Sho 55-145717 discloses a composition comprising an epoxy (meth)acrylate resin and a melamine resin.
Japanese Patent Application, Second Publication No. Sho 50-4395 and Japanese Patent Application, Second Publication No. Sho 53-10636 disclose acrylic resin compositions such as sulfomethylene acrylate or phosphoric acid ethylene acrylate.
Japanese Patent Application, First Publication No. Sho 63-221172 discloses a composition containing polyaminobismaleimide and an epoxy resin as a principal component, Japanese Patent Application, First Publication No. Hei 1-121364 discloses a composition containing polyimide as a principal component, and Japanese Patent Application, First Publication No. Hei 1-256515 discloses a composition containing polyparabanic acid and an epoxy resin as a principal component, respectively.
However, the insulating protective coating film obtained by using the cover lay ink method has poor pliability, heat resistance, and electrical insulating properties, and sometimes has poor adhesion to the circuit board. Therefore, when using the insulating protective coating film in a thin circuit board such as an flexible printed circuit board (hereinafter referred to as a xe2x80x9cFPC boardxe2x80x9d), there is a problem in that warp (hereinafter referred to as xe2x80x9ccurlingxe2x80x9d sometimes) of the FPC board is caused by a difference in thermal expansion coefficients between the circuit board and the insulating protective coating film and curing shrinkage during the formation of the insulating protective coating film.
For example, a coating film made of a composition disclosed in Japanese Patent Application, First Publication No. Sho 55-145717 has the drawback that it has poor pliability.
Coating films made of compositions disclosed in Japanese Patent Application, Second Publication No. Sho 50-4395 and Japanese Patent Application, Second Publication No. Sho 53-10636 were noticeably inferior in heat resistance.
Since all compositions disclosed in Japanese Patent Application, First Publication No. Sho 63-221172, Japanese Patent Application, First Publication No. Hei 1-121364 and Japanese Patent Application, First Publication No. Hei 1-256515 exhibit a large shrinkage factor during curing, severe curling occurs when a cover lay ink coating film is formed on an FPC board by using such a composition, or when the board on which the coating film was formed is subjected to high heat.
No material has ever been found which has a beautiful appearance and pliability, is superior in photosensitivity and developability, and also satisfies performance requirements such as heat resistance, electrical insulating properties, and adhesion to the circuit board, and which does not cause curling even when used in a thin circuit board such as an FPC board.
Under these circumstances, the present invention has been made, and an object thereof is to provide a photosensitive composition which can simultaneously satisfy performance requirements with respect to formation of the photosensitive coating film (e.g. photosensitivity, developability, etc.) and performance requirements for the insulating protective coating film (e.g., heat resistance, electrical insulating properties, adhesion to the circuit board, etc.), and which is capable of forming a cured film having pliability, hardness, warp resistance and good appearance.
As a result of earnest research in order to overcome the above-mentioned problems, the present inventors discovered that a photosensitive composition which contains a photocurable component containing a urethane (meth)acrylate compound (A) having a carboxyl group; a thermosetting resin (c); a photopolymerization initiator (D); and a thermopolymerization catalyst (E); wherein the above-mentioned photocurable component further contains at least one of a compound (B) having an ethylenically unsaturated group, excluding the component (A), and an epoxy (meth)acrylate compound (F) having a carboxyl group, is able to simultaneously satisfy the above-mentioned performance requirements, and when used as a coating material for a circuit board such as an FPC board, it forms an insulating protective coating film which has extremely superior characteristics not found in the conventional art, and thereby the present inventors achieved the present invention.
In more detail, the photosensitive composition of the present invention contains a photocurable component containing a urethane (meth)acrylate compound (A) having a carboxyl group; a thermosetting resin (C); a photopolymerization initiator (D); and a thermopolymerization catalyst (E); wherein the above-mentioned photocurable component further contains at least one of a compound (B) having an ethylenically :unsaturated group, excluding the component (A), and an epoxy (meth)acrylate compound (F) having a carboxyl group.
Ink of the present invention comprises the photosensitive composition and a colorant.
The method of curing a photosensitive composition of the present invention comprises applying the photosensitive composition or ink on a board in a thickness of 10 to 100 xcexcm; drying at a temperature ranging from 60 to 100xc2x0 C. for 5 to 30 minutes, thereby to reduce the thickness to 5 to 70 xcexcm; and heat-curing the photosensitive composition or ink after exposure and development.
The photosensitive dry film of the present invention comprises a support and a photosensitive layer formed from the photosensitive composition on the support.
The method of producing a photosensitive dry film of the present invention comprises the step of applying the photosensitive composition on a support, and drying the photosensitive composition to form a photosensitive layer.
The insulating protective coating film of the present invention comprises the photosensitive composition.
The printed circuit board of the present invention comprises the insulating protective coating film.
The method of producing a printed circuit board of the present invention comprises a laminating step of laminating a photosensitive layer of the photosensitive dry film and a board, an exposure step of exposing the photosensitive layer to light, a developing step which follows the exposure step, and a heat-curing step of heat-curing the photosensitive layer.